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Homework Help: Px = P cos θx equation question.

  1. Dec 1, 2011 #1
    [tex]P_x = P cos θ_x [/tex]

    is Px always the adjacent line of the triangle?
  2. jcsd
  3. Dec 1, 2011 #2
    Yes, if the adjacent side is of a right triangle, where P is the hypotenuse and θ is the angle between the Hypotenuse and adjacent side.
  4. Dec 1, 2011 #3
    Thank you. And a new question.

    Why would it be Px for adjacent and on the other side be P without a subscript?
  5. Dec 1, 2011 #4


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    Staff: Mentor

    Presumably a vector of magnitude P is being decomposed into its X and Y components, with θx being the angle between P and the X axis. Thus:

    Px = P cos(θx)


    Py = P sin(θx)


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    Last edited: Dec 1, 2011
  6. Dec 1, 2011 #5
    Your problem tells me that Px is the component of P in the x-direction. The component of P in the y-direction would be labeled Py. P is the hypotenuse. P might be a force at some angle θ from the x-axis and you are trying to find the component of that force in the x-direction which utilizes the Cosine trig function. If you were trying to find component of the force in the y-direction, Py you would use the sine trig function.
  7. Dec 1, 2011 #6
    So would you ever really have the θy?
  8. Dec 1, 2011 #7
    Yes, look at gneil's image. θx is measured from the x-axis. θy would be the angle measured from the y-axis and would equal 90 degrees- θx.
  9. Dec 1, 2011 #8


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    Staff: Mentor

    Sure. You never know which angle may be given to you or which one might be determined by some other factor in the problem. Example: problems that specify angles for pendulum strings which reference the vertical direction, not the horizontal.
  10. Dec 1, 2011 #9
    Thanks a lot guys. Really! I am in a self study of physics and trigonometry. I am studying electronics and it seems that physics and trig would be a good thing to learn. :)
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